Method for separating substances from a gaseous medium by dry adsorption

ABSTRACT

The treatment, through a dry adsorption process, of a gas from a hot electrolytic process for aluminum production comprises at least two stages. Particulate aluminum oxide (the adsorbent) passes through the stages of the adsorption process countercurrently to the gas. Thus, the gas is treated with a partly spent adsorbent in a first dry adsorption stage, whereupon the particulate adsorbent is separated from the gas downstream from the first adsorption stage. Part of the separated particulate adsorbent is removed from the adsorption process for recycling adsorbed fluorine-containing substances to the electrolytic process. The remainder of the separated adsorbent is recirculated in the first adsorption stage in order to optimize the adsorption of fluorine-containing substances and the desorption of sulfur dioxide from the aluminum oxide in this stage. Simultaneously, the gas is transferred to a second dry adsorption stage. In this second stage, the gas is treated with essentially unspent, reactive particulate aluminum oxide, so that any gaseous fluoride remaining in the gas is very efficiently adsorbed, while at the same time a substantial part of the sulfur dioxide in the gas is also adsorbed. Finally, this particulate aluminum oxide is separated from the gas downstream from the second dry adsorption stage, before the gas is discharged into the surrounding atmosphere. The separated aluminum oxide is transferred to the first adsorption stage, optionally after passing a desorption stage for the removal of the adsorbed sulfur dioxide in order to reduce the discharges of sulfur dioxide from the aluminum production. The separation of sulphur dioxide in the second adsorption stage is improved by recycling, to the second adsorption stage, aluminum oxide having undergone the desorption treatment.

This application is a national stage application filed under 35 U.S.C.§371 of PCT/SE95/01392 filed Nov. 22, 1995.

FIELD OF THE INVENTION

This invention relates to a method for separating, by adsorption and forrecovery purposes, impurities, such as fluorine-containing gases andfluorine-containing dust, from a gas that is emitted from a process foraluminium production. The gas emitted from the process is brought intocontact with an adsorbent in the form of particulate aluminium oxide,which can be recycled as raw material to the process. To be morespecific, the invention concerns a multi-stage countercurrent processcombining effective cleaning of the gas with a high degree ofconcentration of fluorine-containing substances on the adsorbent. In anenvironmentally advantageous embodiment of the invention, sulphurdioxide is simultaneously removed from the gas.

DESCRIPTION OF THE PRIOR ART

In a process for electrolytic production of aluminium, such as theHall-Heroult process where aluminium is produced by reducing aluminiumoxide in a melted electrolyte in the form of a fluoride-containingmineral to which aluminium oxide is supplied, the process gases areloaded with fluorine-containing substances, such as hydrogen fluorideand fluorine-containing dust. Being extremely damaging to theenvironment, these substances have to be separated before the processgases can be discharged into the surrounding atmosphere, while at thesame time the fluorine-containing melt is essential to the electrolyticprocess.

The recovery of fluorine-containing compounds from gases generated inaluminium production suffers from the inconvenience that the process gasusually is loaded also with other substances, such as sulphur dioxide,which originate chiefly from the oxidation of electrodes but to someextent also from impurities found in the raw material. If recycled tothe process along with the adsorbent, these substances will be emittedto the process gases and thus be concentrated in a cycle arising in theelectrolytic process and the gas treatment. If concentrated in theprocess, these substances often have an adverse effect on the yield ofthe process or interfere with the process in some other way, therebyadversely affecting the process economy. Consequently, these substancesshould be removed from the adsorbent before this is recycled to theprocess. For environmental reasons, the amount of sulphur dioxidedischarged from the process should be reduced.

It is previously known to use dry adsorption processes for cleaninggases generated in aluminium production, in which case aluminium oxidemay be used as adsorbent. Aluminium oxide (Al₂ O₃), which as rawmaterial is supplied to the process for aluminium production, has agreat capacity to adsorb (more specifically, to chemically adsorb)hydrogen fluoride. Aluminium-oxide powder of commercial qualities and ofa particle size in the range of 0.03-0.16 mm has a porous structure andan active surface of 40-80 m² /g, such that large amounts of hydrogenfluoride can be adsorbed before the aluminium oxide is saturated. It is,however, true that the adsorption capacity diminishes when the activesurface is all but covered by adsorbed hydrogen-fluoride molecules, i.e.when the aluminium oxide is saturated with hydrogen fluoride. Usually,particulate aluminium oxide is efficiently and turbulently mixed withthe gases from the aluminium-production process in a fluidised bed orsome other contact reactor, hydrogen fluoride being then adsorbed on thealuminium oxide. The aluminium oxide, which now contains adsorbedfluorides, is separated downstream from the contact reactor with the aidof one or more filters. The aluminium oxide is then supplied to thealuminium-production process, and the fluorides are recovered. However,also sulphur dioxide is to a certain extent (as a rule 10-30%) adsorbedin these processes, and the sulphur dioxide thus accompanies thealuminium oxide back to the aluminium-production process, where it isreleased to the process gases in the furnace. In actual practice, thesulphur dioxide thus is not removed from the gases, but is instead inundesirable fashion recycled and concentrated in a system including thealuminium-production furnace and the gas-cleaning equipment, and onefurther obtains an increase of the sulphur-dioxide content in the air onthe premises. If one wishes to reduce the environmentally-hazardousdischarge of sulphur dioxide from the aluminium production, the sulphurdioxide has to be separated from the flue gases with the aid of wetseparators arranged downstream from prior-art dry adsorption processes.However, such wet separators used for separating sulphur dioxide fromthe gases represent a very expensive solution, since the amounts of gasinvolved are considerable and the sulphur-dioxide concentration thereinis low, for instance as compared with that of flue gases from afossile-fired power plant. For this reason, most of the world'saluminium plants still discharge all the sulphur dioxide into thesurrounding atmosphere.

One object of this invention is to provide a method for, with the aid ofdry adsorption on aluminium oxide, separating essentially all thefluorine-containing substances for recovery purposes, as well asefficiently separating sulphur dioxide for environmental reasons, from agas emitted from a process for aluminium production.

Another object of the invention is to provide a process by means ofwhich sulphur dioxide and, to a certain extent, also other undesirableimpurities on the adsorbent can be removed therefrom before theadsorbent is recycled to the process for aluminium production, therebyto avoid recirculation and accumulation of these substances in thesystem.

Yet another object of the invention is to provide a process which, incomparison with prior-art dry gas-cleaning processes, results inmaintained or improved separation and recovery of fluorine-containingsubstances, while maintaining or improving the environmental-friendlycharacter of the process (low emissions), as compared with the prior-artprocesses mentioned above.

DESCRIPTION OF THE INVENTION

According to the invention, these objects are achieved by an adsorptionprocess which comprises at least two dry adsorption stages, in which agas, which is generated in a process for aluminium production and isloaded at least with fluorine-containing substances that may be gaseousor particulate, is mixed with and brought into contact with particulatealuminium oxide, thereby to separate at least the fluorine-containingsubstances from the gas. The adsorption stages are arranged in the formof one or more contact reactors, in which the gas is treated by beingmixed and contacted with particulate aluminium oxide.

In the adsorption process according to the invention,

the gas is treated in a first dry adsorption stage with at least partlyspent particulate aluminium oxide, such that a substantial part of thegaseous fluorides found in the gas are adsorbed on the adsorbent,

the aluminium oxide with adsorbed fluorine-containing substances isseparated from the gas downstream from the first adsorption stage,whereupon part of the separated aluminium oxide with adsorbedfluorine-containing compounds is removed from the adsorption process,and the remainder of the aluminium oxide is recirculated in the firstadsorption stage, while at the same time the gas is transferred to asecond dry adsorption stage arranged downstream from the firstadsorption stage,

the gas now having a substantially reduced content offluorine-containing substances is then, in the second dry adsorptionstage, treated with essentially unspent reactive aluminium oxide inparticulate form, thereby to adsorb any fluorine-containing substancesremaining in the gas after the first adsorption stage and to adsorbother gases, such as sulphur dioxide, and

the particulate aluminium oxide is then removed from the gas downstreamfrom the second adsorption stage, before the gas is discharged into thesurrounding atmosphere or undergoes a supplementary treatmentdownstream, and at least part of the aluminium oxide separated from thegas downstream from a contact reactor included in the second adsorptionstage is transferred to a contact reactor included in the firstadsorption stage.

As appears from the foregoing, the particulate aluminium oxide passesthrough the stages of the adsorption process countercurrently to thegas. The unspent aluminium oxide is first supplied to a contact reactorwhich is included in the second dry adsorption stage and where thealuminium oxide is mixed with and brought into contact with the gas.From the contact reactor included in the second adsorption stage, atleast some of the now partly spent aluminium oxide is transferred to acontact reactor included in the first adsorption stage. When supplied toa contact reactor included in the first dry adsorption stage, aluminiumoxide from the second adsorption stage is mixed with and brought intocontact with the gas in this first adsorption stage. After passingthrough a contact reactor included in the first adsorption stage, partof the dry particulate aluminium oxide is separated, the aluminium oxidebeing now essentially saturated at least with fluorine-containingsubstances and being removed from the process, thereby to recyclefluorine-containing substances to the process for aluminium production,the remainder of the aluminium oxide being recirculated in the firstadsorption stage.

This recirculation is motivated by two reasons. First, one wishes tocontrol and optimise the adsorption of gaseous fluoride from the processgas in the first adsorption stage. Second, one wishes to obtain theaimed-at desorption of such substances as sulphur dioxide, which havebeen adsorbed on the aluminium oxide in the second adsorption stage,thereby to prevent any substantial recycling of these substances to theelectrolytic process. Should sulphur (sulphur dioxide) or phosphorus(phosphorus pentoxide) be recycled to the electrolytic process, thismight have an adverse effect on the yield of this process.

Since aluminium oxide has a much higher affinity with hydrogen fluoridethan with such gases as sulphur dioxide, it is possible, by partlyrecirculating the adsorbent in at least the first adsorption stage, tocheck which substances are recycled to the electrolytic process alongwith the adsorbent transferred to the electrolysis furnace, thuspreventing undesirable substances, such as sulphur dioxide andphosphorus pentoxide, from being recirculated and concentrated in thesystem including the electrolysis furnace and the gas-treatmentequipment. All such gases are adsorbed and molecularly bound to theactive surface of the oxide particle in a dry adsorption process. Since,however, hydrogen fluoride has a higher affinity with the oxide thanwith sulphur dioxide, the sulphur dioxide already adsorbed will bedesorbed, while hydrogen fluoride takes the place of the sulphur dioxideon the active surface. Under excellent contact conditions betweenprocess gas and adsorbent, the adsorption process strives towards astate of equilibrium with a very high proportion of adsorbed hydrogenfluoride on the oxide surface, where adsorbed sulphur dioxide onlyoccurs if there is an excess of active adsorbent surface in relation tothe amount of hydrogen fluoride present in the process. Owing to thefact that the adsorbent is recirculated in the first adsorption stage,the process approaches this state of equilibrium. As a result, theadsorption of undesirable substances can be monitored and minimised,such that it is only a minimum of these substances that is recycled tothe electrolysis furnace together with the adsorbent.

In one embodiment of the invention intended for use when one wishes toavoid that undesirable substances, such as sulphur dioxide, that havebeen adsorbed on the adsorbent are recycled to the electrolysis furnace,but one nevertheless may allow these substances to be discharged intothe surrounding atmosphere, the adsorbent (aluminium oxide) istransferred from the second adsorption stage directly to the firstadsorption stage, where it is recirculated while sulphur dioxide isdesorbed. The desorption is guided towards a state of equilibrium.Sulphur dioxide is emitted from the electrolysis process and accompaniesthe process gas to the first adsorption stage. However, the adsorptionof sulphur dioxide in this first stage is controlled and minimisedthrough adsorbent recirculation in this stage. As a result, the sulphurdioxide will be concentrated in a cycle between the first and the secondadsorption stage, whereas essentially no sulphur dioxide will berecirculated between the electrolysis furnace and the first adsorptionstage. At steady state, a state of equilibrium finally establishesitself, in which the amount of sulphur dioxide discharged into thesurrounding atmosphere equals the amount of sulphur dioxide emitted tothe process gases in the electrolysis furnace.

DRAWING

For exemplifying purposes, the invention will now be described in moredetail with the aid of a preferred embodiment, reference be had to theaccompanying drawing.

DESCRIPTION OF THE DRAWING

In the Hall-Heroult process, aluminium is produced by reducing aluminiumoxide, which is dissolved in a melt of fluorine-containing minerals,with the aid of electrolysis in an electric reduction furnace 1. Theelectrolysis takes place at a temperature of approximately 960° C. Themelt is partly disintegrated during the process, and volatile componentsleave in gaseous state. As a result, the gases emitted from the processcontain fluorine compounds, such as hydrogen fluoride (HF) andfluorine-containing dust. Being extremely damaging to the environment,these substances have to be separated from the process gases beforethese can be discharged into the surrounding atmosphere. At the sametime, however, these fluorine-containing substances represent a loss ofconsiderable value. Apart from the fluorine-containing compounds, thereare present certain combustion products, such as sulphur dioxide, fromthe carbon anodes, which are combusted during the process. Sulphurdioxide should be removed from the adsorbent not only in order to avoidthat it is recycled to the process, but also because it is desirable,for environmental reasons, to reduce the discharges of sulphur dioxidefrom the process without having to set up vast and costly plants fortreating the large amounts of gas having a low content of sulphurdioxide.

When the invention is used for treating a gas 2 emitted from a process 1for aluminium production, fluorine-containing substances are separatedfrom the gas in a countercurrent adsorption process comprising at leasttwo dry adsorption stages 3, 4. The gas loaded with fluorine-containingsubstances is treated in a first dry adsorption stage 3, which in theFigure is shown as a contact reactor 3. In this contact reactor 3, thegas is mixed with and brought into contact with a partly spentparticulate adsorbent in the form of aluminium oxide, which is conveyedwith the current of gas in the contact reactor 3, the content offluorine-containing substances in the process gas being reduced. Theadsorption of sulphur dioxide during the treatment in the firstadsorption stage 3, when the content of fluorine-containing substancesin the gas is at its highest, is suppressed since such substances ashydrogen fluoride have a much greater affinity with aluminium oxide thanwith sulphur dioxide. In this first adsorption stage, sulphur dioxide isthus only adsorbed on excess surface on the aluminium oxide, which isnot covered by e.g. hydrogen fluoride. If aluminium oxide on whichsulphur dioxide has been adsorbed comes into sufficiently intensivecontact with gas containing hydrogen fluoride, the sulphur dioxide willbe released and replaced by hydrogen fluoride. After the treatment inthe first adsorption stage 3, the particulate aluminium oxide isseparated from the gas before the latter, now having a very low contentof hydrogen fluoride, is transferred to a second dry adsorption stage 4for treatment there. The particulate aluminium oxide, which has a highcontent of adsorbed fluorine-containing substances, such as hydrogenfluoride, is separated from the gas along with the main part of theparticulate fluorine compounds downstream from the first adsorptionstage 3 with the aid of prior-art mechanical separating devices 31, suchas cyclones. Some aluminium oxide 33, which corresponds to the amount ofunspent aluminium oxide supplied to the second adsorption stage 4 of theadsorption process and which is loaded with adsorbed fluorine-containingsubstances, is (at 33) recycled to the process 1, while the remainder ofthe aluminium oxide is recirculated (at 32) within the first adsorptionstage 3. Through sufficient recirculation and owing to the difference inthe affinity of the aluminium oxide with respectively hydrogen fluorideand sulphur dioxide, it is ensured that the main part of thefluorine-containing substances in the gas are adsorbed even at the firstadsorption stage 3, whereas there is essentially no adsorption ofsulphur dioxide. Instead, a substantial amount of the sulphur dioxide 1adsorbed on the aluminium oxide is desorbed. As a result, essentiallyall the sulphur dioxide will accompany the gas, so that thefluorine-containing substances vital to the process 1 can be recycledwith a good yield at 33, while avoiding the recirculation andconcentration of sulphur dioxide in the process. Also the secondadsorption stage 4 is arranged in the form of one or more contactreactors 4 disposed downstream from the first adsorption stage 3. Fromthe first adsorption stage 3 and the following separator 31, the gas istransferred at 30 to the contact reactors 4, where it is mixed with andbrought into contact with fresh, reactive and essentially unspentaluminium oxide. In the contact reactor 4, any remaining gaseousfluorine, as well as sulphur dioxide, is adsorbed in an amount dependingon the extent to which the adsorption capacity of the fresh adsorbent(aluminium oxide) allows adsorption of low-affinity gas. After thetreatment in the second adsorption stage 4, the adsorbent is separatedfrom the gas with the aid of a filter 41, such as a bag filter,whereupon the gas, which has been very efficiently cleaned of allfluorine-containing substances, can be discharged into the surroundingatmosphere at 5, whereas the aluminium oxide loaded with a substantialamount of the sulphur dioxide adsorbed in the second adsorption stage 4is, in accordance with the invention, transferred to the firstadsorption stage 3. Through suitable recirculation of aluminium oxide 32in the adsorption stage 3, a substantial amount of the sulphur dioxideadsorbed on the aluminium oxide will, when contacted with process gashaving a higher content of hydrogen fluoride, be desorbed in theadsorption stage 3. The released sulphur dioxide is then conducted alongwith the process gas to the second adsorption stage 4. Owing to thedesorption of sulphur dioxide, there is an increase in the activesurface on the aluminium oxide that is available to hydrogen fluorideadsorption, resulting in a highly efficient adsorption of hydrogenfluoride, such that a very high degree of adsorption of gaseous fluorineis obtained in the first adsorption stage 3.

Through the aluminium oxide 33 that is transferred to the reductionprocess 1 from the first adsorption stage 3, substantially allfluorine-containing substances emitted from the reduction process 1 tothe process gas 2 are recycled to the reduction process 1. However,essentially no sulphur dioxide is recycled to the reduction process 1along with the aluminium oxide 33 transferred from the first adsorptionstage 3 to the reduction process 1.

Owing to the fact that sulphur dioxide is desorbed in the firstadsorption stage, the partly cleaned process gas 30 transferred to thesecond adsorption stage 4 will have an increased sulphur-dioxidecontent, which to some extent is reduced in the second adsorption stage4. At steady state, a state of equilibrium is established as regards therecirculation of concentrated sulphur dioxide between the two adsorptionstages 3 and 4, the amount of sulphur dioxide discharged along with thecleaned process gas 5 equalling the amount of sulphur dioxide suppliedtogether with the as yet uncleaned process gas.

In one embodiment of the invention, also the sulphur dioxide dischargedto the surrounding atmosphere together with the cleaned process gas 5 isreduced by treating the sulphur-dioxide-loaded aluminium oxide from thesecond adsorption stage 7 in a desorption stage 8. In this desorptionstage 8, substantially all the adsorbed sulphur dioxide is desorbed bybeing heated and mixed with a through-flowing carrier gas 81. Thecarrier gas 82 leaving the desorption stage 8 will then have a highconcentration of sulphur dioxide, essentially all the sulphur dioxidehaving been emitted from the aluminium oxide through desorption.

Owing to the low affinity of sulphur dioxide with aluminium oxide, thealuminium oxide has a fairly restricted capacity to adsorb sulphurdioxide. One thus obtains, even at such a low content of gaseousfluoride in the process gas 30 transferred to the second adsorptionstage 4 as has an essentially negligible effect on the sulphur-dioxideadsorption during this adsorption stage 4, a poor separation of sulphurdioxide from the process gas, if the adsorbent quality is low and/or ifthe sulphur-dioxide content of the process gas supplied to this secondadsorption stage 4 is high. In one embodiment of the invention, thecapacity to separate sulphur dioxide is raised to the aimed-at level byrecycling (at 83) part of the aluminium oxide treated in the desorptionstage 8 to the second adsorption stage 4, where it contributes to anincrease in the amount of active adsorbent. The amount of aluminiumoxide treated in the desorption stage 8 will thus increaseproportionally to the amount of aluminium oxide recycled at 83 from thedesorption stage 8 to the second adsorption stage 4.

In the desorption stage 8, the sulphur dioxide is desorbed owing to theeffect of heating and through-flowing carrier gas 81, entraining thesulphur dioxide with it on its way out of the system. If the desorptiontreatment in stage 8 is correctly performed, only a small amount ofcarrier gas 81 is required, while at the same time a high concentrationof sulphur dioxide is obtained in the carrier gas 82 leaving thedesorption stage. The sulphur dioxide in the carrier gas can, at areasonable cost, be washed or converted to commercial products, such asliquid sulphur dioxide, sulphuric acid or sulphur, by using well-knownprocesses, since there is only a small amount of carrier gas 82involved, for which reason the treatment equipment may be of small size.The slight heating of the aluminium oxide required in order to desorbthe sulphur dioxide in the desorption stage 8 does not result in adesorption of the small amount of hydrogen fluoride that has beenadsorbed in the second adsorption stage 4. After the desorption stage 8,the aluminium oxide is conducted to the first adsorption stage 3, as hasbeen described in the foregoing.

After having thus passed the dry two-stage adsorption process 3, 4countercurrently to the gas and adsorbed substantially all the hydrogenfluoride and other fluorine-containing substances from the gas, thealuminium oxide is supplied to the process 1 for aluminium production.The sulphur-dioxide content of the aluminium oxide is very low and isessentially limited to the amount that has been adsorbed and hasremained during the treatment in the first adsorption stage 3. Certainother substances, such as phosphorus, which have been entrained with thegas from the aluminium-production process 1 and which reduce the currentyield in the electrolytic process, have an adverse effect on the processand should therefore be removed. Phosphorus in the form of particulatephosphorus pentoxide is removed from the process gas in a finalfiltration stage 41 and may thus be concentrated in the system includingthe electrolysis furnace and the gas-treatment equipment. It has beenfound that the treatment for the removal of sulphur dioxide 8 alsoremoves a certain amount of phosphorus, thereby reducing theaccumulation thereof in the system.

Since, in accordance with the inventive method, the particulateadsorbent (the aluminium oxide) passes the two stages 3, 4 of theadsorption process countercurrently to the gas, whereas the gas and theadsorbent are jointly conveyed with the current in the adsorption stages3, 4, the adsorbent is efficiently spent and essentially all thehydrogen fluoride separated in the first adsorption stage 3 and recycledalong with the adsorbent to the aluminium-production process 1, whilesulphur dioxide is separated in the second adsorption stage 4 and isremoved from the adsorbent in the desorption stage 8. The separation ofsulphur dioxide can be adjusted to the aimed-at efficiency through therecirculation at 83 of the adsorbent from the desorption stage 8 to thesecond adsorption stage 4. This two-stage process results in recyclingwith a high yield of the fluorine-containing substances that one wishesto recirculate from the process, whereas the sulphur dioxide can beseparated by itself and either be neutralised in an alkali scrubber orbe recovered in the form of commercially viable products. Since theprocess according to the invention in its simplest form reduces therecirculation and accumulation of sulphur dioxide and in its moreelaborate form also reduces the recirculation and accumulation of such apollutant as phosphorus in the aluminium-production process, there isachieved an improved efficiency in the electrolytic process foraluminium production, since this process would otherwise be adverselyaffected by increasing contents of these substances. Since sulphur canbe separated in one embodiment of the invention, the environmentalfriendliness of aluminium production in its entirety can be improved.

We claim:
 1. A method for separating fluorine-containing substances from a gas emitted from a process for aluminum production by means of adsorption on solid, particulate aluminum oxide in a dry adsorption process, comprising:treating said gas in a first dry adsorption process with partly spent particulate aluminum oxide to absorb fluorine-containing substances, said aluminum oxide passing countercurrently to said gas; separating the particulate aluminum oxide with adsorbed fluorine-containing substances from said gas downstream from said first adsorption stage, before said gas is transferred to a second dry adsorption stage; removing part of the separated particulate aluminum oxide with adsorbed fluorine-containing substances from the adsorption process and recycling said part of separated particulate aluminum oxide with fluorine-containing substances to the process for aluminum production, the remainder of the separated aluminum oxide being recirculated in the first adsorption stage; and after separation of said aluminum oxide, supplying said gas to a second dry adsorption stage and treating said gas with essentially unspent reactive particulate aluminum oxide, said unspent aluminum oxide passing countercurrently to said gas, whereupon the particulate aluminum oxide is separated from the gas downstream from the second dry adsorption stage, before said gas is discharged into the surrounding atmosphere, and at least part of the aluminum oxide separated downstream from the second adsorption stage is transferred to the first adsorption stage.
 2. A method as claimed in claim 1, wherein the amount of partly spent aluminum oxide recirculated is monitored and so controlled as to optimize the adsorption of fluorine-containing substances onto the aluminum oxide and the desorption of sulfur dioxide from the aluminum oxide in the first adsorption stage.
 3. A method as claimed in claim 2, wherein said gas contains at least hydrogen fluoride and sulfur dioxide, and wherein the aluminum oxide separated downstream from the second adsorption stage contains sulfur dioxide adsorbed from said gas, and treating said aluminum oxide with adsorbed sulfur dioxide in a desorption stage where the aluminum oxide is heated and a carrier gas flows through it, thereby desorbing a substantial amount of the sulphur dioxide adsorbed on the aluminum oxide.
 4. A method as claimed in claim 3, wherein part of the aluminum oxide treated in the desorption stage is recycled to the second adsorption stage in order to enhance the adsorption capacity in this stage.
 5. A method as claimed in claim 4, wherein water vapor or nitrogen gas flows through the aluminum oxide in the desorption stage.
 6. A method as claimed in claim 1, wherein said gas contains at least hydrogen fluoride and sulfur dioxide, and wherein the aluminum oxide separated downstream from the second adsorption stage contains sulfur dioxide adsorbed from said gas, and treating said aluminum oxide with adsorbed sulfur dioxide in a desorption stage where the aluminum oxide is heated and a carrier gas flows through it, thereby desorbing a substantial amount of the sulphur dioxide adsorbed on the aluminum oxide.
 7. A method as claimed in claim 6, wherein part of the aluminum oxide treated in the desorption stage is recycled to the second adsorption stage in order to enhance the adsorption capacity in this stage.
 8. A method as claimed in claim 7, wherein water vapor or nitrogen gas flows through the aluminum oxide in the desorption stage.
 9. A method as claimed in claim 6, wherein water vapor or nitrogen gas flows through the aluminum oxide in the desorption stage. 